1. Field of the Invention
The present invention relates to a process for preparing an oil-in-water type emulsion of a vinyl-type polymer by emulsion polymerization of a polymerizable vinyl monomer and using a polyester having a surface active property as an emulsifier.
2. Description of the Prior Art
As conventional processes for the preparation of synthetic resin emulsions, there are known a process in which a monomer is emulsion-polymerized using as an emulsifier a low-molecular-weight surface active agent such as polyoxyethylene nonyl phenyl ether and a process in which emulsion polymerization is carried out by using as a protective colloid a water-soluble polymeric compound such as polyvinyl alcohol or polyvinyl pyrrolidone. In general, dry films made of synthetic resin emulsions prepared by using low molecular-weight surface active agents as emulsifiers are poor in water resistance, and the presence of the surface active agents inhibits adhesion of the films to coated substrates and/or degrades the mechanical strength of the films. In the case of emulsions obtained by the aid of a protective colloid of a water-soluble polymeric compound, the size of the dispersed particles is comparatively large, and when the emulsions are used as paint vehicles, they show poor pigment dispersibility and the resulting coatings are inferior in gloss, coloring property and adhesion to substrates.
In paints, adhesives and binders comprising organic solvents, serious problems such as pollution of air and bad effects on the health of workers are caused by the organic solvents, and recently, attempts have been made to replace the conventional solvent-type film-forming materials by aqueous-type film-forming materials. In the field of emulsion polymerization techniques, investigations have been made to utilize and develop new materials as emulsifiers. For example, Japanese Patent Application Laid-Open Specification No. 43381/74 proposed a process for the preparation of vinyl-type polymer emulsions using maleic acid-modified butadiene as an emulsifier, and Japanese Patent Application Laid-Open Specification No. 128089/74 disclosed a process for the preparation of aqueous dispersion-type resin compositions using a specific water-soluble alkyd resin as an emulsifier.
These processes, however, are defective or insufficient in various points. For example, because of the low emulsifying powers of the emulsifiers used, the conditions needed for obtaining stable emulsified polymers are severely limited. Further, the large quantities of watersoluble polymeric compounds that are used tend to increase the emulsion viscosity and make it difficult to control the flowability of the compositions.